1. Field of the Invention
The invention relates to a paint spray gun, more particularly to a paint spray gun that can improve efficiency of a paint spraying operation.
2. Description of the Related Art
Referring to FIGS. 1 and 2, a conventional paint spray gun 1 is shown to include a gun body 11, an operating unit 12, and a spray cap 13.
The gun body 11 has a handle portion 111, a head portion 112 connected to the handle portion 111, and a nozzle 113 mounted on one end of the head portion 112. The nozzle 113 is formed with a nozzle orifice 114. The head portion 112 is formed with a material passageway 115 that is in fluid communication with the nozzle orifice 114 and that provides a path for flow of paint material from a paint canister (not shown) to the nozzle orifice 114. The nozzle 113 is further formed with a first air passageway 116 that is fluidly isolated from the nozzle orifice 114. The gun body 11 is further formed with a second air passageway 117 that extends from the handle portion 111 and along the head portion 112, that is in fluid communication with the first air passageway 116, and that provides a path for flow of pressurized air from a compressed air source (not shown) to the first air passageway 116.
The operating unit 12 is mounted on the gun body 11, and includes a material flow controlling valve 121 for controlling the flow of the paint material through the nozzle orifice 114, an air flow controlling valve 123 for controlling the flow of the pressurized air through the second air passageway 117, a trigger 122 coupled operably to and controlling opening and closing actions of the material flow controlling valve 121 and the air flow controlling valve 123, and an air flow regulator 124 for regulating air flow through the second air passageway 117.
The spray cap 13 includes a cap body 131. The cap body 131 is disposed in front of the nozzle 113, and is formed with a central spray hole 132, two pairs of mist forming holes 133, and a pair of horn projections 134. The central spray hole 132 has a hole axis (z) extending in a longitudinal direction, and is registered and in fluid communication with the nozzle orifice 114. The mist forming holes 133 in each pair are opposite to each other in a diametrical direction (r) relative to the hole axis (z), and are disposed on opposite sides of the central spray hole 132. The horn projections 134 extend from the cap body 131 in the longitudinal direction away from the nozzle 113, and are disposed adjacent to the mist forming holes 133. Each horn projection 134 is formed with a third air passageway 1340. The third air passageway 1340 terminates in an outlet opening 135 that is formed in a lateral side, which faces toward the central spray hole 132, of the corresponding horn projection 134. The mist forming holes 133 and the third air passageways 1340 are in fluid communication with the first air passageway 116. Since the mist forming holes 133 are formed in a convex wall part of the cap body 131, the mist forming holes 133 do not appear to be disposed on a straight line in FIG. 2, but are actually disposed on a curved line and are aligned in the diametrical direction (r).
In use, when the trigger 122 of the operating unit 12 is operated to open the air flow controlling valve 123, pressurized air from the compressed air source (not shown) flows through the second air passageway 117, the first air passageway 116, and exits through the mist forming holes 133 and the outlet openings 135, thus creating a negative air pressure around the central spray hole 132. Since operation of the trigger 122 also results in opening of the material flow controlling valve 121, the paint material is drawn out of the paint canister (not shown) via the material passageway 115, and exits through the central spray hole 132, where it will be atomized by the pressurized air stream that exits through the mist forming holes 133 and the outlet openings 135 such that the paint material can be sprayed on a target surface.
However, due to the high velocity of the pressurized air stream, a portion of the atomized paint material is likely to bounce off the target surface, thereby polluting the work environment and resulting in wasted paint material.
Referring to FIG. 3, based on actual tests using the conventional paint spray gun 1, when the air stream pressure at the vicinity of the central spray hole 132 ranges between 27 to 40 psi, only 35% to 40% of the atomized paint material adheres to the target surface to form an effective painted area (A). The remainder of the atomized paint material is either scattered to the ambient air, or spread on the target surface within a spread distance (D) from the effective painted area (A) to form an ineffective painted area (B).
Referring to FIG. 4, when the air stream pressure at the vicinity of the central spray hole 132 is reduced to below 10 psi, such as by operating the air flow regulator 124 or by using a high volume low pressure (HVLP) paint spray gun, up to 60% of the atomized paint material adheres to the target surface to form the effective painted area (A), and the amount of the atomized paint material that scatters to the ambient air or that forms the ineffective painted area (B) is accordingly reduced. However, due to the reduction in the air stream pressure, the atomizing efficiency is lowered such that the painted area has a coarse surface and lacks luster.
U.S. Pat. No. 3,252,657 discloses a spray gun air cap provided with a central orifice and a pair of cap orifices respectively disposed on opposite sides of the central orifice. The spray gun air cap further has a pair of lugs on opposite sides of the cap orifices. Each lug is provided with a conduit, a lug orifice for directing air into the stream of fluid emanating from the central orifice, and a set of orifices for spreading air fanwise parallel to the fluid stream to form an air envelope to minimize over-spray. However, since the lugs, the cap orifices and the central orifices are aligned along a first diametrical direction relative to the central orifice, control of over-spray along a second diametrical direction transverse to the first diametrical direction is poor.
U.S. Pat. No. 2,070,696 discloses a spray head formed with a central orifice, a pair of first orifices on opposite sides of and aligned with the central orifice in a first diametrical direction, and a pair of second orifices on opposite sides of and aligned with the central orifice in a second diametrical direction transverse to the first diametrical direction. However, air streams coming out of the first and second orifices are for atomizing a spray stream from the central orifice and do not form an air barrier that could minimize scattering of the spray stream and reduce over-spray.